This invention relates to electrical engineering and to capacitor engineering in particular, and can be used for manufacturing of high capacitance capacitors utilizing the energy of the electric double layer (EDL). EDL capacitors have found their use as backup power sources in systems requiring uninterrupted electric power supply, such as computers, communication devices, digital programmable lathes, continuous production cycles; for electric starting of internal combustion engines, powering the engines of wheelchairs, golf carts, etc.
Several electric power storage devices exist in the form of electric double layer (EDL) capacitors, for example, as described in U.S. Pat. No. 4,313,084 (1982) and U.S. Pat. No. 4,562,511 (1985). Such capacitors consist of two porous polarizable electrodes with a porous separator made of a dielectric material between them, and current collectors. A liquid electrolyte, which can be either non-aqueous or aqueous, including an aqueous sulfuric acid solution, is retained in the pores of the electrodes and the separator and also fills some free volume inside the capacitor case. The electric charge is accumulated in the pores on the interface between the electrode material and the electrolyte. Various porous carbonaceous materials are normally used for manufacturing of polarizable electrodes. To increase the capacitance of the electric double layer capacitor, these carbonaceous materials are subjected to prior activation for the purpose of increasing their specific surface area up to 500-3000 m2/g.
EDL capacitors have much higher capacitance than conventional electrostatic and electrolytic capacitorsxe2x80x94up to tens or hundreds of farads per gram of active electrode material. However, a disadvantage of these capacitors is their rather low specific energy, which does not exceed 3 Wh/l. This maximum value of specific energy for double-layer capacitors is set with non-aqueous electrolytes, where the maximum voltage values are in the range of 3 to 3.5 V. However, such capacitors permit relatively low discharge and charge currents due to the lower conductivity of the non-aqueous electrolytes. Still lower specific energies, 0.5 to 2 Wh/l, have been achieved by double-layer capacitors employing aqueous electrolytes with maximum voltage value of approximately 0.9 V. When such double-layer capacitors remain under charge for a prolonged period of time (which is often quite long) at voltages higher than 0.9 V, noticeable oxidation, i.e., corrosion, of the positive carbon electrode takes place accompanied by evolution of oxygen and carbon dioxide.
An electric double layer capacitor having only one polarizable electrode made of a carbonaceous material, is described in Patent of Japan, Accepted Application No 2-11008. The other electrode is non-polarizable, i.e., battery type, made of lithium or lithium alloy. The electrolyte is non-aqueous. Such a capacitor has higher specific energy compared to the conventional double-layer capacitor with two polarizable electrodes. However, a drawback of this prototype is the very low charge and discharge current (0.1 to 1 mA/cm2) and, therefore, very low power density due to the use of a non-aqueous electrolyte. Another essential disadvantage of the rechargeable device in question is its very low cycleabilityxe2x80x94about 100-200 cycles.
An EDL capacitor with only one polarizable electrode made of a fibrous carbonaceous material is described in Patent WO 97/07518 [4]. The other electrode, made of nickel oxide, is non-polarizable. An aqueous solution of alkaline metal carbonate or hydroxide serves as electrolyte. Such a capacitor excels considerably the double-layer capacitors with two polarizable electrodes in both specific energy (up to 12.5 Wh/l) and maximum voltage (1.4 V).
However, this capacitor has a number of shortcomings:
1) Insufficiently high specific energy;
2) High cost, due to the use of large amounts of nickel oxide.
Still another drawback of the EDL capacitors is the gas generation on the electrodes at overcharge, e.g. of oxygen on the positive electrode and/or hydrogen on the negative electrode. This occurs when the evolution potentials of these gases on the corresponding electrodes are reached at overcharge. As a result, the pressure within the capacitor case increases, which can lead to its decompression and even explosion, unless it is equipped with a special pressure relief valve. But even such valves often are not reliable enough to prevent decompression or explosion: they can, for instance, become clogged with dirt, etc. On account of all this, EDL capacitors have a fundamental disadvantage: the possibility of their decompression and even explosion and need of special maintenance. In order to prevent decompression, the end-of-charge voltage is significantly reduced for reinsurance, thus reducing the initial discharge voltage as well. This, in its turn, leads to a considerable decrease in the EDL capacitor specific energy, which is proportional to the difference between the squares of the initial and final discharge voltages.
Application PCT/RU97/00411 relates to an EDL capacitor having lead sulfate as an active mass of the non-polarizable positive electrode.
U.S. Pat. No. 6,195,252 relates to an EDL capacitor having lead dioxide as an active mass of the non-polarizable electrode. The capacitors described in PCT/RU 97/10041 and U.S. Pat. No. 6,195,252 have are advantageous over that described in WO 97/07518 in their considerably higher maximum voltage of 2.1 V and the correspondingly higher values of specific energy.
The capacitors described in PCT/RU 97/0041 and U.S. Pat. No. 6,195,252 have the following common disadvantages: high cost, insufficient cycle life, and average values of specific energy of up to 40 Wh/l.
The capacitor described in U.S. Pat. No. 6,195,252 is considered hereafter as the closest in terms of both design and performance to the one described in the present invention.
The objects of the present invention are to increase the cycle life and specific energy of the capacitor and to reduce its cost.
These objects are achieved by the invention described below. In accordance with the invention, a capacitor is provided, which comprises a polarizable electrode made of a porous carbon material, a non-polarizable electrode based on lead sulfate and lead dioxide as active components, and an aqueous solution of sulfuric acid as electrolyte, whereas the mass ratio of any of the active components to their sum ranges from 0.1 to 99% by weight.
The following reversible electrochemical reaction takes place during discharge and charge on the positive electrode:
PbO2+HSO4xe2x88x92+3H+2ePbSO4+2H2Oxe2x80x83xe2x80x83(1) 
Under the working conditions used, the maximum equilibrium potential of this reaction (which depends on the concentration of the electrolyte) in the charged state is approximately 1.9 V.
During cycling, the EDL on the negative electrode is recharged. The recharge of the EDL can be described as follows:
(H+)ad/Cxe2x88x92+HSO4xe2x88x92H++(HSO4xe2x88x92)ad/C++2exe2x88x92xe2x80x83xe2x80x83(2). 
Here the designation (H+)ad/Cxe2x88x92 refers to a proton adsorbed in the EDL on the negatively charged surface of the carbon electrode (for the charged state of the capacitor), and (HSO4xe2x88x92)ad/C+ to a bisulfate ion adsorbed in the EDL on the positively charged surface of the carbon electrode (for the discharged state of the capacitor). In our experiments, the potential of said electrode changed in the range from xe2x88x920.2 to 1.0 V vs. normal hydrogen electrode in the same solution. Thus, during a charge-discharge process the potential changes in the range form xe2x88x920.6 to +2.1 V.
Combining of equations (1) and (2) gives the overall equation of the electrochemical reaction taking place in the electrical double layer capacitor, described herein:
PbO2+2H2SO4+(H+)ad/Cxe2x88x92PbSO4+2H2O +(HSO4xe2x88x92)ad/C+xe2x80x83xe2x80x83(3) 
It should be noted that the active mass of the positive electrode contains both lead dioxide and lead sulfate at any degree of discharge. This has been demonstrated by the results of the chemical analysis. As such, at fully charged state the actual mass ratio PbSO4/(PbO2+PbSO4) is generally not lower than 0.1%, while at fully discharged state it is generally not higher than 99%. Therefore, the requirements on the purity of the active mass as formulated in both PCT/RU 97/0041 and U.S. Pat. No. 6,185,252 are unrealistic, since they envisage the use of pure PbSO4 and pure PbO2, respectively.
The capacitor disclosed herein is considerably (by approximately 30 to 60%) cheaper than those disclosed in PCT/RU 97/0041 and U.S. Pat. No. 6,185,252. This is due to the avoided expenses for purifying the active mass (PbSO4 and PbO2, respectively), which is not necessary according to the present invention. Moreover, the tri-component non-polarizable electrode containing PbO2, PbSO4, and PbO provides better performance of the capacitor.
In a preferred embodiment of the present invention, the non-polarizable electrode contains also lead monoxide, PbO, ranging from 0.2 to 5% of the mass of the lead sulfate. The addition of this small amount of lead monoxide modifies the structure of the active mass in such a way that the degradation of the porous structure of said electrode is diminished at not very small currents. For best performance, not only ideal reversibility of the electrochemical processes in the capacitor is necessary, but also ideal reversibility of the changes in the porous structure of the positive electrode. In practice, however, during multiple cycling the porous structure gradually degrades. This brings about decrease in the capacity of said electrode and corresponding decrease in the capacity and specific energy of the capacitor as a whole. Addition of small amounts of PbO slows down the degradation processes at not very low currents. As a result, the cycle life increases by 20 to 30%. When the cycle number is equal, the specific energy increases.
In another preferred embodiment of the present invention, the polarizable electrode contains a small amount of lead in addition to the carbon. The desirable lead content of the polarizable electrode ranges from 0.03 to 3% by weight. The addition of small quantities of lead to the active mass of the polarizable electrode causes significant increase in its capacity (by 100-300 F) due to changes in the surface groups and improved hydrophilization of the carbon surface, and thereby, to increase in the working surface of this electrode. Furthermore, by considerably lowering the rate of several electrochemical reactions responsible for electrolyte decomposition, the lead additive reduces the leakage currents and extends the cycle life of the capacitor.
In still another preferred embodiment of the present invention, the non-polarizable electrode contains also lead phosphate, Pb3(PO4)2, ranging from 0.1 to 5% of the active mass. Such small addition of lead phosphate reduces the sulphation rate of said electrode and diminishes guttering, thereby improving the cycleability by 20-50%.
The implementation of the present invention results in higher by far specific capacity, specific energy and cycle life, as well as in much lower cost.
An important advantage of the capacitor disclosed herein over a lead-acid battery is that the former is more adaptive to complete sealing. This arises from two main reasons:
1. Even at minimum potential of the carbon electrode (Ec=xe2x88x920.2 V vs. hydrogen electrode in the same solution), which is reached at end of charge, hydrogen is not generated due to the high overpotential of hydrogen evolution on carbon, especially when small amount of lead is present in the carbon electrode.
2. Oxygen that can be generated at end of charge on the positive electrode can be fully reduced on the negative electrode since the activated carbon has good catalytic ability for this process, especially when the very high maximum polarization of 1.4 V is taken into account. Activated carbon, for instance, is applied for electroreduction of oxygen in fuel cells [Bagotsky V. S., Skundin A. M., Chemical Current Sources, Energy, 1981]. Thus, there is no need of additives such as platinum, palladium, etc., which facilitate burning of oxygen and hydrogen in some lead-acid cells [Patent of Japan, Accepted Application No. 60-35475], thereby enabling the sealing of the latter.